1. Field
Embodiments of the invention relate to a method for preparing treated electrolytic manganese dioxide (EMD). More specifically, certain embodiments of the invention are directed to a method and a system for preparing treated EMD to prevent metal-containing particulates formed, for example, during the milling or other handling of the EMD from causing problems, such as an internal short in a battery.
2. Description of the Related Art
EMD is commonly used as an active material for dry battery cells because it is an inexpensive and abundant material and it provides excellent discharge and long-term storage performance. For example, EMD is used as a cathode-active material in a primary alkaline or lithium battery or as a precursor for an active material of a positive electrode in a lithium-ion battery.
EMD is typically prepared by passing a direct current through an acidic solution of manganese sulfate and sulfuric acid. The positive electrode of this plating may include a plate of titanium onto which the EMD is deposited. The negative electrode may be made of graphite or copper, or similar material. The deposited EMD is mechanically removed from the titanium plate after it has reached a thickness of about 1 mm to about 75 mm. The EMD pieces removed from the titanium plate are reduced in size to meet the requirements of the battery manufacturer using a grinding or milling process. The resulting EMD product is referred to as milled EMD.
The size reduction of the EMD pieces generally occurs in a mill. Because of the hardness of the EMD, the mill frequently uses steel parts to grind the EMD to a specific size. During the milling process, these steel parts may wear, causing metallic impurities to be introduced into the EMD. In some cases, the impurities may not be a piece of steel; rather the impurities may include a piece of rust or other contaminant. These impurities will hereinafter be referred to as metal-containing particulates.
Because EMD is prepared in an acidic bath, the preparation of EMD generally requires a washing and/or caustic treatment of the EMD to neutralize the acidity of the bath. This treatment may be performed before or after the milling of the EMD.
In a final step, the EMD is dried to certain specifications. For example, for primary alkaline-battery applications, the drying step is generally mild, leaving behind the chemically bound water and physisorbed water, which may range from about 1% to about 3% of the product weight. The resulting material is the active material for primary alkaline batteries.
For primary-lithium-battery applications, the active material is prepared by removing all water, whether chemical or physisorbed, to avoid any reaction with the organic electrolyte and/or the metallic lithium in the battery.
When EMD is used as a precursor for an active material of a positive electrode in lithium-ion batteries, EMD powder is reacted with a lithium salt (e.g., lithium carbonate) and potential dopants at temperatures between 600° C. and 900° C.
None of these steps for preparing the EMD, which may occur after the milling process, are designed to remove the metal-containing particulates.
The metal-containing particulates included with the positive electrode of the battery may, in the presence of an electrolyte, be converted to dissolved metal ions. The ions may migrate to the negative electrode, where they may be reduced back to a metal. Subsequent ions may also be reduced on the surface of this metal that is in contact with the negative electrode. Through repeated depositions, a metallic chain may develop that leads through the pores of the separator, causing an internal short in the battery. This internal short will, in a best case scenario, slowly discharge the battery and render the battery inoperable. In a worst case scenario, the internal short will rapidly discharge the battery and could generate significant heat, potentially causing a battery containing organic electrolytes to vent, catch fire, or even explode.
The generation of the internal short is directly linked to a large number of metal atoms that are concentrated within a small volume within the positive electrode of the battery. These atoms, after dissolution into ions, will seek a short path to the negative electrode and reconstitute as a metallic impurity within a similarly small volume of the separator. This high concentration of metal atoms near, and eventually in, the separator causes the battery to short. If the same number of atoms was more widely distributed in the positive electrode, an insufficient number of metal atoms would coalesce on the negative electrode to form a conductive path to the positive electrode, thereby preventing the internal short.
The metal-containing particulates that have been identified as causing a majority of internal shorts in batteries include Fe for lithium-ion batteries and Cu for primary alkaline batteries.